Plastic materials such as thermoplastic olefin (TPO) and reaction injected molding urethane (RIM) have been developed which are useful in many applications, such as automobile parts and accessories, containers, household appliances and other commercial items. It is often desirable to coat articles made from such plastic materials with organic coating compositions to decorate them or to protect the plastic materials from degradation when exposed to atmospheric weathering conditions such as sunlight, moisture, heat and cold. To achieve longer lasting and more durable parts, it is necessary for the coatings to be tightly adhered to the surface of the plastic.
Plastic substrates made from a variety of thermoplastic and thermosetting plastic materials have widely varying surface properties, including surface tension, roughness and flexibility, which make it difficult to achieve adequate adhesion of the organic coatings to such materials, particularly after aging or environmental exposure of the plastic materials. It is well known that to properly adhere coatings to some plastic materials, an adhesion promoter or tie coat can be used. The application of an adhesion promoter or tie coat is normally an added step in the coating process. The adhesion promoter is usually applied in a thin layer, normally about 0.25 mils (6.35 microns (.mu.m)). Typically, adhesion promoters used on TPO plastic surfaces contain chlorinated polyolefins, some examples of which are described in U.S. Pat. Nos. 4,997,882; 5,319,032 and 5,397,602. Additionally, flame or corona pretreatment steps can be used to facilitate adequate adhesion of organic coatings to some plastic substrates.
The use of adhesion promoters and corona pretreatments in a coating system used to coat plastic substrates adds complexity and cost to the system. The application of an adhesion promoter usually entails coating the plastic substrate with the promoter, followed by some drying or curing time which increases the time of the entire coating process and will usually necessitate additional workspace. Accordingly, coating compositions which exhibit excellent adhesion directly to plastic materials such as TPO and RIM without the use of adhesion promoters or tie coats are desirable.
Polyolefin diols have been used in coating compositions to impart adhesion to the plastic substrate without the use of adhesion promoters or tie coats. However, polyolefin diols may be incompatible with the resins and/or crosslinking agents used in some coating compositions.